A gas turbine engine has a number of rotor assemblies. Each rotor assembly has a plurality of rotor stages having a plurality of rotating blades. The blades are surrounded by a case. The blades and the case are separated by a clearance. The clearance between the blades and the case is a critical factor in the performance of the engine. Too small of a clearance results in rubbing between the blades and the case, and thereby reduces engine efficiency and engine life. Too large of a clearance reduces engine efficiency, and in a compressor, can lead to a compressor stall condition.
It is desirable to be able to determine the magnitude of the clearance in order to control the clearance or compensate for clearances that are too large or too small. Methods presently exist to adequately determine a steady state clearance for an engine operating condition, i.e., the clearance that exists when the rotor, the blades, and the case are at a thermal equilibrium condition. However, some gas turbine engines undergo a wide range of operating conditions. A transition from one operating condition to another operating condition introduces a thermal non-equilibrium condition in the rotor, the blades, and the case. The clearance during the thermal non-equilibrium condition can be significantly larger or significantly smaller than the steady state clearance for the engine operating condition. This difference can persist until the rotor, the blades, and the case each reach the thermal equilibrium condition for the engine operating condition. Thus, methods are needed to adequately determine the clearance during thermal non-equilibrium conditions as well.
U.S. Pat. No. 5,012,420 discloses a system to calculate the instantaneous clearance between a turbine casing and a turbine rotor based on temperature. Steady state temperatures for the casing and the rotor are computed based on presently occurring engine operating conditions such as selected temperatures, pressures, and rotational speeds. Instantaneous temperatures are computed based on changes in the steady state temperatures. The system calculates the instantaneous clearance during steady state and non-steady state conditions. However, the need to calculate the steady state temperatures and the instantaneous temperatures of the casing and the rotor makes this system complex and therefore difficult to a particular gas turbine engine.
U.S. Pat. No. 5,165,845 discloses a system for controlling stall margin by synthesizing the thermal enlargement of critical compressor stages to synthesize the temporary increase in blade-case clearance during acceleration. This system does not require calculation of the steady state temperatures or the instantaneous temperatures of the rotor, the blade, and the case. However, the synthesis of the temporary increase blade-case clearance does not include effects related to a temporary difference that results from a difference between the steady state clearance before the acceleration and the steady state clearance after the acceleration. Consequently, this system can be difficult to adapt from one type of gas turbine engine to another type of gas turbine engine. In addition, this system has limited flexibility with respect to representing thermal expansion characteristics of the rotor, the blade, and the case.